1. Field of the Invention
The present invention relates to a filter device, such as an antenna duplexer, for example, including at least one filter chip mounted on a substrate formed by stacking a plurality of ceramic layers.
2. Description of Related Art
As shown in FIG. 10, an antenna duplexer includes an antenna terminal ANT to be connected to an antenna, a transmission side signal terminal Tx to be connected to a transmitting circuit, and a reception side signal terminal Rx to be connected to a receiving circuit. The antenna terminal ANT is connected to the transmission side signal terminal Tx via a transmission filter chip 2 including a surface acoustic wave device, and connected to the reception side signal terminal Rx via a reception filter chip 3 including a surface acoustic wave device (see JP 11-340781, A). A phase-matching strip line 6 for phase rotation is interposed between the antenna terminal ANT and the reception filter chip 3 to match the phases between the transmission filter chip 2 and the reception filter chip 3 (see JP 2000-307383, A).
FIG. 11 and FIG. 12 show the construction of the antenna duplexer enclosed in a package. A cavity 80 is provided on a surface of a substrate 8 formed by stacking a plurality of ceramic layers. The transmission filter chip 2 and the reception filter chip 3 are mounted on the bottom surface of the cavity 80. A plurality of terminals provided on these filter chips are respectively coupled by wires 4 to a plurality of pads 7, 71 arranged around. As shown in FIG. 12, a plurality of side electrodes 95, 96 for serving as a plurality of external terminals of the transmission side signal terminal Tx, the reception side signal terminal Rx, etc. are formed on side surfaces of the substrate 8. The plurality of pads 7, 71 are connected via signal wiring patterns 90, 91 and the side electrodes 95, 96 to foot terminals 93, 94 formed on the reverse surface of the substrate 8. The cavity 80 of the substrate 8 is closed by a lid 50.
FIG. 13 to FIG. 18 show six ceramic layers 81-86 of the substrate 8. In FIG. 11 to FIG. 18, reference characters Tx, Rx, ANT, A, B, C and D represent parts of the transmission side signal terminal Tx, the reception side signal terminal Rx, the antenna terminal ANT, input and output terminals A, B of the transmission filter chip 2 and input and output terminals C, D of the reception filter chip 3, respectively, shown in FIG. 10. Further, a reference character G represents a part of ground terminals. As shown in FIG. 13, a top first ceramic layer 81 has a cavity 80a provided at the middle thereof. As shown in FIG. 14, a cavity 80b is provided at the middle of a second ceramic layer 82. A plurality of pads 7, 71, 72, 73 to be wire-bonded with a plurality of terminals of the transmission filter chip 2 and the reception filter chip 3, ground patterns 43, 44, and a plurality of signal wiring patterns 90, 91 are formed on a surface of the ceramic layer 82, surrounding the cavity 80b. As shown in FIG. 15, a ground pattern 41 is formed on a surface of a third ceramic layer 83. The transmission filter chip 2 and the reception filter chip 3 are to be mounted on the ground pattern 41. As shown in FIG. 16, the phase-matching strip line 6 is formed on a surface of a fourth ceramic layer 84. As shown in FIG. 17, a ground pattern 42 is formed on a surface of a fifth ceramic layer 85. Further, as shown in FIG. 18, a plurality of foot terminals 93, 94 are formed on the reverse surface of a bottom sixth ceramic layer 86.
The transmission side signal terminal Tx shown in FIG. 14 is connected through the transmission side input signal wiring pattern 91 to the pad 71, and further connected via a wire to the signal input terminal A of the transmission filter chip 2. The signal output terminal B of the transmission filter chip 2 is coupled via a wire to the pad 72, and connected to the antenna terminal ANT. The pad 72 is connected to one terminal E of the phase-matching strip line 6 shown in FIG. 16 through a conductive material in a via hole (hereafter simply referred to as a via) 5 provided in the second ceramic layer 82 and the third ceramic layer 83 in FIG. 15. The other terminal F of the phase-matching strip line 6 is connected to the pad 73 shown in FIG. 14 through a via 51 provided in the third ceramic layer 83 in FIG. 15 and the second ceramic layer 82 in FIG. 14. The pad 73 is coupled by a wire to the signal input terminal C of the reception filter chip 3. The signal output terminal D of the reception filter chip 3 is coupled to the pad 7 by a wire. The pad 7 is connected through the reception side output signal wiring pattern 90 to the reception side signal terminal Rx. The reception side output signal wiring pattern 90 has an inductance component, which improves signal pass characteristics in a signal pass band of the reception filter chip 3.
As shown in FIG. 14, two ground terminals G, G at the signal input side and signal output side of the reception filter chip 3 are connected via wires to the signal input side ground pattern 44 and the signal output side ground pattern 43, respectively.
Further, the ground pattern 41 shown in FIG. 15 and the ground pattern 42 shown in FIG. 17 are connected to each other via a plurality of vias 51 provided in the fourth ceramic layer 84 as shown in FIG. 16.
In the antenna duplexer, during signal transmission, signals input to the transmission side signal terminal Tx pass through the transmission filter chip 2, and are thereafter output from the antenna terminal ANT to the antenna. At this time, it is necessary to prevent a phenomenon of the signals input to the transmission side signal terminal Tx flowing into the reception side signal terminal Rx.
Accordingly, for the conventional antenna duplexer, various isolation measures have been taken on a signal path from the transmission side signal terminal Tx through the transmission filter chip 2 and the reception filter chip 3 to the reception side signal terminal Rx. However, the conventional antenna duplexer has not yet achieved a sufficient isolation effect, but left room for further improvement.